The present invention relates to an induction motor having a single one-piece rotor and a plurality of stators which can start smoothly and output a high driving torque in a wide range from a low speed range to a high speed range and, more particularly, to a phase changing system for the motor which effects a phase shifting operation in the rotating magnetic fields around the rotor conductive members of the rotor, generated by the stators surrounding the rotor.
As for the methods for controlling the torque and speed in an induction motor of the type having a plurality of stators, a conventional method available is to cause phase differences between the stators. A mechanical system for effecting such phase differences may have an arrangement wherein the relative rotation of stators is relied upon to produce a phase difference and, an electrical system therefor may have an arrangement wherein the connections of stator windings are changed to produce several kinds of phase differences. Other available systems include one wherein Y-.DELTA.-connection changing system is combined.
Any appropriate method is selected in a number of different ways from among the above methods or systems according to the load characteristics of load connected to the motor or the use of the motor, such as the need to meet the load by changing the torque or the speed of the motor and the need to make a smooth increase in the speed at the starting of the motor.
The present invention provides an arrangement in which certain stepwise phase differences are provided to meet the load connected and which may be regarded as falling within the electrical systems for effecting phase changes as referred to above in the explanation of the conventional methods.
In the conventional electrical systems explained above, the phase changing is effected by the connection-changing of the stator windings, so that, while the phase differences available can be of 0.degree., 60.degree., 120.degree., 180.degree. in the electrical angles, the number of the switches required will exceed a dozen or more. This is one of the reasons for the high manufacturing cost of such system.
There is also an ordinary induction motor in which a Y-.DELTA.-connection changing system is provided for the purpose of improving the starting characteristics of the motor. In such a motor, despite the provision of only one stator, its wirings are quite complicated. Also, there occurs a torque change caused by a temporary interruption of the load current at the time when Y to .DELTA. connection changing takes place, and further, there occurs an unavoidable shock caused by a sudden increase in the load current and by a sudden change in the generated torque immediately after the above connection changing. The torque-speed and current-speed characteristic curves obtained according to the conventional Y-.DELTA. connection changing are shown in FIG. 14.